Phase-locked loops have been used for many years. They are often used in clock recovery circuits in the receivers of digital systems for the purpose of synchronizing the local clock with the incoming data signal. The data signal is processed so as to provide synchronizing information to a phase detector which is an element of the phase-locked loop. Detection apparatus of the type shown in U.S. Pat. No. 4,535,459, assigned to the same assignee as the present invention may be used for the combined purposes of clock recovery and data detection. The above apparatus combines the functions of data processing and phase detection into a single circuit but, for the purpose of discussion, those functions may be thought of as separate functions. The phase detection function works as well or better than other phase detectors but it is not a phase/frequency detector and, therefore, it requires that the frequency and phase of the local clock (VCO) be within the lock range of the phase-locked loop in order for phase lock (synchronization) to be acquired. A phase-locked loop of this type therefore requires some means of determining when the loop is out of lock so some means can be used to assist the local clock (VCO) to be brought quickly to within the lock range of the loop.
Prior art phase-locked loops have utilized an embedded oscillator circuit in series between the phase detector and the voltage-controlled oscillator of a phase-locked loop where the embedded circuit is transparent to normal phase lock operation and where the embedded circuit commences oscillating when the loop looses lock and at this time attempts to drive the voltage-controlled oscillator back to an operational frequency such that lock can again occur. Such an approach has been practiced using a Wien-bridge oscillator. A unique feature of the Wien-bridge oscillator which permitted such an operation was that if its gain was less than three, the oscillations would die out. When a phase-locked loop is in the locked condition, the gain of any circuits within the loop is reduced so that in the locked condition the oscillator is not oscillating. Thus, this circuit provides a desirable function. The negative or undesirable part of this approach is that the Wien bridge oscillator has a sine wave output only if the amplifier's gain is exactly three. As the gain exceeds three, the amplifier goes rapidly towards a square wave output. Thus, there are severe production problems in maintaining a gain of three. Temperature and aging problems also contribute to the unusability of the circuit for many applications. Further, the resulting waveform, which is used to sweep the VCO, has a high slew rate as it crosses zero. This can cause the VCO to enter, pass through, and leave the locked range in less than the lockup time of the phase-locked loop.
If the oscillator time constants are set for a frequency of more than a few hertz it will introduce peaking in the response of the phase-locked loop and such peaking may exceed customer defined specifications for situations where the present invention is to be used in series with other circuits having similar transfer functions.
The present invention is designed to eliminate these problems by preventing the oscillating signal generated by the embedded circuit from reaching the VCO, and instead using an oscillation detector to detect a voltage generated within the embedded circuit when the embedded circuit is oscillating. An output is provided from the detector to a switch circuit that controls whether or not a sweep signal is supplied to the voltage-controlled oscillator.
It is therefore an object of the present invention to provide an improved lock reacquisition circuit in a phase-locked loop.